Compressor discharge bleed air circuit in gas turbine plants and related method

ABSTRACT

A gas turbine system that includes a compressor, a turbine component and a load, wherein fuel and compressor discharge bleed air are supplied to a combustor and gaseous products of combustion are introduced into the turbine component and subsequently exhausted to atmosphere. A compressor discharge bleed air circuit removes bleed air from the compressor and supplies one portion of the bleed air to the combustor and another portion of the compressor discharge bleed air to an exhaust stack of the turbine component in a single cycle system, or to a heat recovery steam generator in a combined cycle system. In both systems, the bleed air diverted from the combustor may be expanded in an air expander to reduce pressure upstream of the exhaust stack or heat recovery steam generator.

This application is a continuation of application Ser. No. 09/659,687,filed Sep. 11, 2000, U.S. Pat. No. 6,442,941 the entire content of whichis hereby incorporated by reference in this application.

This invention was made with Government support under Contract No.DE-FC21-95MC31176 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

In some gas turbine applications, there are instances of gas turbineplant operation where the gas turbine pressure ratio reaches theoperating pressure ratio limit of the compressor, resulting incompressor surge. These instances may arise in applications wherelow-Btu fuels or any other fuels with large amounts of diluent injectionare used, and/or at cold ambient temperature conditions. The compressorpressure ratio is typically larger than the turbine pressure ratio inthat the latter is subject to pressure loss in the turbine combustor.

One common solution that has been used to provide compressor pressureratio protection is the bleeding off of gas turbine compressor dischargeair and recirculating the bleed air back to the compressor inlet. Thismethod of gas turbine operation, known as Inlet Bleed Heat (IBH)Control, raises the inlet temperature of the compressor inlet air bymixing the colder ambient air with the bleed portion of the hotcompressor discharge air, thereby reducing the air density and the massflow to the gas turbine. While this approach eliminates compressorsurge, it also reduces turbine output both for the simple cycleoperation as well as for combined cycle operation. In the latter case,the reduced gas turbine exhaust flow produces less steam in the HeatRecovery Steam Generator (HRSG) and consequently less steam turbineoutput. IBH also reduces the thermal efficiency of the gas turbine dueto the loss of energy in throttling the compressed air.

BRIEF SUMMARY OF THE INVENTION

This invention provides an improved compressor bleed air method forproviding compressor pressure ratio protection, which results inimproved output and efficiency of a simple or combined cycle gas turbinepower plant (as compared to the IBH approach). This invention is mostly,but not specifically, applicable to gas turbines utilizing standarddiffusion flame combustors.

Two embodiments are disclosed herein. Each has applicability to bothsimple and combined cycle systems.

In a first embodiment, the invention includes bleeding off enough gasturbine compressor discharge air to maintain the compressor pressureratio limit, and mixing it with the gas turbine (GT) exhaust in a simplecycle system, or at an appropriate location in a combined cycle system(e.g., in the HRSG stack where the two streams have minimum temperaturedifference). This technique does not increase compressor inlettemperature, and thus does not reduce output as in the case of the IBHapproach.

In a second embodiment, where the compressor bleed function is activatedfor a large percentage of the gas turbine operating period, the methodis similar to that described above, except that it uses an air expanderdevice to recover the excess energy associated with the differencebetween the compressor air discharge pressure and GT exhaust stack (orHRSG) pressure. In addition to the power output increases, this methodalso results in higher power plant efficiency. In this secondembodiment, a portion of the compressor discharge bleed air may bypassthe expander via a throttling device to combine with the dischargestream from the expander, to thereby enable plant operation during startup, shut down and during the events when the expander is not operating.

The following are additional optional modifications which may beselected (individually or in an appropriate combination, in both simpleand combined cycle operations) based on the economic benefits for agiven application. The high pressure bleed air from the compressor isfurther heated, if required, by means of a pre-heater prior tointroduction in the air expander to improve the expander output. Thesource of this heat can be thermal energy recovered either upstream,such as in the example case of a gasifier with high temperature cooler,or downstream such as the exhaust gas heat recovered from the gasturbine exhaust in a waste heat boiler. Alternatively, the source ofheat may include combustion of air and fuel separately supplied to thepre-heater.

In its broader aspects, therefore, the invention relates to a simplecycle gas turbine system comprising: a compressor, a turbine componentand a load, wherein fuel and compressor discharge air are supplied to acombustor and gaseous products of combustion are introduced into theturbine component and subsequently exhausted to atmosphere; and acompressor discharge bleed air circuit that removes bleed air from thecompressor and supplies one portion of the bleed air to the combustorand another portion of the bleed air to an exhaust stack of the turbinecomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a simple cycle gas turbine withcompressor bleed air to an exhaust stack in accordance with theinvention;

FIG. 2 is a schematic diagram of a simple cycle gas turbine withcompressor bleed air pressure energy recovery in accordance with theinvention;

FIG. 3 is a schematic diagram of a combined cycle gas turbine withcompressor bleed air to a heat recovery steam generator; and

FIG. 4 is a schematic diagram of a combined cycle gas turbine withcompressor bleed air pressure energy recovery.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, the simple cycle gas turbine system 10includes a compressor 12, a turbine component 14 and a load (e.g., agenerator) 16 arranged on a single rotor or shaft 18. A combustor 20 ofthe gas turbine receives fuel via stream 22 and control valve 24, aswell as hot discharge air bled off from the compressor 12 via stream 26.Combustion gases are introduced into the turbine component 14 via stream28.

During potential compressor surge conditions, a compressor dischargebleed air circuit is utilized. This circuit causes some of thecompressor discharge bleed air to bypass the combustor and directs thisbleed air directly to the gas turbine exhaust stack 30 via stream 32 andthrottle valve 34, the valve 34 also controlling the amount of dischargeair introduced into the combustor 20.

By extracting sufficient compressor discharge bleed air and feeding itdirectly to the gas turbine exhaust stack 30, the compressor pressureratio limit is protected while, at the same time, there is no increasein the compressor inlet air temperature, and thus no loss of turbineoutput.

In FIG. 2, an arrangement is illustrated that is particularly beneficialwhen the compressor bleed air function is employed for a largepercentage of the gas turbine operating period. In this embodiment, thegas turbine system 36 includes a compressor 38, a turbine component 40and a load (e.g., a generator) 42, arranged on a single rotor or shaft44. The combustor 46 receives fuel via stream 48 and fuel control valve50; and compressor discharge air from the compressor 38 via stream 52.Combustion gases are introduced into the turbine component 40 via stream54. A predetermined percentage of the compressor discharge air is bledto a flow control/bypass valve 56. During potential compressor surgeconditions, valve 56 supplies compressor discharge bleed air to an airexpander 68 via stream 70, and the air representing the differencebetween the compressor air discharge pressure and the gas turbineexhaust pressure, is then used to drive a secondary load 72 (e.g., agenerator) via shaft 74. Optionally, the valve 56 can adjustably divertthe compressor discharge bleed air to the gas turbine exhaust stack 58via stream 60 and flow control throttle valve 62. This is useful as abypass scheme (bypassing expander 68) to continue plant operation duringstart-up, shut-down and other events when the expander is not operating.

In an alternative arrangement, valve 56 may supply the compressordischarge bleed air to a pre-heater 64 via stream 66. Pre-heater 64heats the bleed air by heat exchange with turbine exhaust air fed to thepre-heater 64 via stream 76. The heated bleed air is then expanded asdescribed above. The pre-heater 64, optionally, may be fired using fuelseparately introduced via stream 78. Excess air from the expander 68 isintroduced into stream 60 via stream 80 and then to the gas turbineexhaust stack 58. Some percentage of this excess air may be allowed tobypass the stack 58 and escape to atmosphere upstream of the stack 58via stream 81 and valve 82.

During potential compressor surge conditions, a portion of thecompressor discharge air is supplied to the HRSG 104 via stream 114 andflow control/throttle valve 116, where it mixes with the gas turbineexhaust before being released to atmosphere via the HRSG exhaust stack118. As in the embodiment shown in FIG. 1, this arrangement does notresult in an increase in compressor inlet air temperature, thus allowingthe compressor to enjoy the full benefit of low ambient temperatures (orother factors that also produce compressor surge).

Turning now to FIG. 4, an arrangement is shown that is applicable tocombined cycle systems and uses an air expander to recover energyassociated with the difference between the compressor discharge airpressure and the HRSG pressure. This combined cycle system 120 includesa gas turbine having a compressor 122, a turbine component 124 and aload (e.g., a generator) 126 arranged on a single shaft 128. Combustor130 receives fuel via stream 132 and fuel control valve 134, along withcompressor discharge air from the compressor 122 via stream 136.Combustion gases from the combustor 130 are introduced into the turbine124 via stream 138. The gas turbine exhaust is supplied via stream 140to an HRSG 142 for reheating steam from the steam turbine 144. Condensedsteam from the steam turbine 144 is supplied to the HRSG 142 via stream146, and the reheated steam is returned to the steam turbine 144 viastream 148. Steam turbine 144 drives a generator 145.

During potential compressor surge conditions, a predetermined percentageof the compressor discharge air is bled to a flow control/bypass valve150 via stream 152. Valve 150 supplies the bleed air to the expander 154via stream 156. Optionally, the bleed air can first be supplied to apre-heater 158 via stream 156. The pre-heater 158 heats the compressordischarge bleed air via heat exchange with gas turbine exhaust in theHRSG via stream 160. The pre-heater 158, optionally, may be fired usingfuel introduced via stream 162. The heated compressor discharge bleedair is then expanded in the air expander 154 and the excess air is usedto drive a third load (e.g., a generator) 164 via shaft 166. Duringstart-up, shut-down or other events when the expander is not operating,the valve 150 may divert the compressor discharge bleed air to the HRSG142 via stream 166 and flow control/throttle valve 168, thus bypassingthe pre-heater 158 and expander 154. When in service, air from theexpander 154 is dumped into the stream 166 upstream of the HRSG 142, viastream 170. It is eventually exhausted to atmosphere through the HRSGstack 172. Some percentage of this air may bypass the HRSG 142 andescape to atmosphere via stream 174 and valve 176.

It is significant that the compressor discharge bleed air circuitsdescribed above are useful under conditions that lead to compressorsurge, i.e., low ambient air temperatures; excess flow to the turbine;use of fuels with low heat content, etc. By channeling the compressorbleed air downstream of the compressor, there is no increase incompressor inlet temperature and attendant loss of input as with the IBHapproach. With higher ambient temperatures, flow is reduced and there istypically no danger of compressor surge, so that the bleed airtechniques of this invention are not required.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A gas turbine system comprising: a compressor, aturbine component and a load, wherein fuel and compressor discharge airare supplied to a combustor and gaseous products of combustion areintroduced into the turbine component and subsequently exhausted toatmosphere; and a compressor discharge bleed air circuit that includes aflow control/bypass valve arranged to supply one portion of thecompressor discharge air to the combustor and another portion of thecompressor discharge air, selectively, to an expander or directly to anexhaust stack of the turbine component.
 2. The system of claim 1 whereinsaid compressor discharge bleed air circuit includes a throttle valvefor controlling flow of said another portion of the compressor dischargeair directly to the exhaust stack.
 3. The system of claim 1 wherein saidcompressor, turbine component and load are on a single shaft.
 4. Thesystem of claim 1 wherein, when said flow control/bypass valve isenabled to divert said another portion of the compressor discharge airto the air expander, said another portion of the compressor dischargeair is introduced into a pre-heater upstream of said expander.
 5. Thesystem of claim 1 wherein, when said flow control/bypass valve isenabled to divert said another portion of the compressor discharge airto the air expander, said another portion of the compressor dischargeair discharged from said expander is introduced into said exhaust stack.6. The system of claim 1 wherein said another portion of the compressordischarge air is bypassed around the expander to enable turbineoperation during start-up and shut-down.
 7. A method of avoidingcompressor surge under low ambient temperature conditions in a gasturbine operating system that includes a compressor, a turbine componentand a load, the method comprising: a. supplying one portion of dischargeair from the compressor to a combustor of the gas turbine; and b.supplying another portion of the compressor discharge air selectively,to an expander upstream of an exhaust stack of said turbine component tothereby avoid compressor surge without increasing compressor inlet airtemperature, or, bypassing said expander, directly to said exhauststack.
 8. The method of claim 7 wherein bypassing said another portionof the compressor discharge air around said expander is implementedduring startup and shut down.
 9. The method of claim 7 including usingsome of the compressor discharge air exiting the expander to driveanother load.